Optical recording and reading system, optical data storage medium and use of such medium

ABSTRACT

An optical recording and reading system for use with an optical data storage medium ( 5 ) is described. The system comprises the medium ( 5 ) having a recording stack ( 9 ), formed on a substrate ( 8 ). The recording stack ( 9 ) is suitable for recording by means of a focused radiation beam ( 1 ) with a wavelength λ in air. The recording stack ( 9 ) has a first optical surface ( 6 ) most remote from the substrate ( 8 ). An optical head ( 3 ), with an objective ( 2 ) having a numerical aperture NA&gt;0.8 and from which objective ( 2 ) the focused radiation beam emanates ( 1 ) during recording, is ( 2 ) arranged on the recording stack ( 9 ) side of said optical data storage medium ( 5 ). The objective has a second optical surface ( 7 ) closest to the recording stack ( 9 ), and is adapted for recording/reading at a free working distance d F  of smaller than 50 μm from the first optical surface ( 6 ). At least one of the first optical surface ( 6 ) and the second optical surface ( 7 ) is provided with a transparent hydrophobic layer ( 10 ) that has a refractive index n and has a thickness smaller than 0.5 vn. In this way reliable recording and reading is achieved, specifically contamination build-up on the second optical surface ( 7 ) is prevented or counteracted.

The invention relates to an optical recording and reading system for usewith an optical data storage medium, said system comprising:

-   -   the medium having a recording stack, formed on a substrate, said        recording stack suitable for recording by means of a focused        radiation beam with a wavelength λ in air, the recording stack        having a first optical surface most remote from the substrate;        and    -   an optical head, with an objective having a numerical aperture        NA and from which objective the focused radiation beam emanates        during recording, the objective arranged on the recording stack        side of said optical data storage medium and having a second        optical surface closest to the recording stack, and adapted for        recording/reading at a free working distance d_(F) of smaller        than 50 μm from the first optical surface.

The invention further relates to an optical data storage medium having arecording stack, formed on a substrate, said recording stack suitablefor recording by means of a focused radiation beam, with a wavelength λin air, the recording stack having a first optical surface most remotefrom the substrate.

The invention further relates to the use of such a medium in such asystem.

An embodiment of a system of the type mentioned in the opening paragraphis known from U.S. Pat. No. 6,069,853.

New generations of optical recording disks have even larger datacapacity and smaller bit sizes. There is a tendency that the wavelengthfor the optical readout decreases and the numerical aperture (NA) of theoptical pick up unit (OPU) increases for each new generation. Focallength and working distance decrease, and tilt margins are become evermore stringent. The result is that the transparent layer through whichthe recording layer is being recorded and read is decreasing inthickness: 1.2 mm for Compact Disk (CD), 0.6 mm for Digital VersatileDisk, and 0.1 mm for Blu-ray Disk (BD) down to the level of a fewmicrons for the “4^(th)-generation” (magneto-) optical disks. Thepurpose of the layer is at least threefold: to make the disk moredurable by protecting the information layer, to serve as ananti-reflection coating and to aid cooling of the storage layer.

For future generations of optical storage systems the numerical apertureof the objective will rise to NA=0.85, or even NA=0.95, to improveresolving power. Despite this tendency of the objective to increase insize, however, the increasing demand for high data rate and access timeforces the total mass of the objective to shrink. This can only beaccomplished if the focal length and hence the free working distance(FWD) is reduced.

However if the FWD is reduced the thickness of the transparentsubstrate, through which the focused radiation beam passes, needs to bereduced. Furthermore, if the NA is increased, then the allowance of theangle by which the medium surface is deviated from the perpendicularwith respect to an optical axis of the optical system (tilt angle) isreduced under the effect of double refraction or aberration due to thethickness of the transparent substrate. Thus reduction of the effect ofthe tilt angle at high NA is another reason to decrease the thickness ofthe transparent substrate. This transparent substrate with reducedthickness is also called cover layer or more generally cover stack. Thusthe purpose of the cover layer in 4^(th) generation optical recording ismainly to protect the recording stack from damage and to enable a lowFWD. To record information in the above-mentioned (magneto-) opticaldata storage medium, a focused radiation beam, e.g. light, is radiatedfrom an optical head through the transparent substrate having athickness between 0.6 to 1.2 mm to the recording stack so as to heat therecording layers to a recording temperature. In case of magneto-optical(MO) media, at the same time, a magnetic field is applied by a magnetichead. Such magnetic field may be modulated with the information by useof a magnetic field modulation device. As a result, the information maybe recorded on the recording medium.

Another argument for a small free working distance is the size of thecoil in case of the magneto-optical data storage medium. If one wants asystem with a high data rate, a large bandwidth to modulate the currentthrough the coil is required. For data rates in the order of 100-200Mbit/sec, the switching frequency of the current through the coil mustbe at least 1-2 GHz, in order to define sharp flanks in the switchingbehavior of the field. This requires a coil with a smallself-inductance, low resistance and small parasitic capacitance. Apartfrom the speed of the coil, the power consumption by the coil is also anissue. Therefore it is preferable to use a small coil with a small innerdiameter, e.g. smaller than 100 μm. The use of a bigger coil wouldcompromise the data rate and energy efficiency because bigger coils havea larger inductance and a higher power consumption. The closer the coilis brought to the surface, the more energy efficient the magnetic fieldat the data storage medium can be modulated. However, a magnetic fieldin the order of 15 kA/m per Ampere of such a small coil penetrates onlya few tens of microns into space, so the coil must be kept close to therecording stack and a cover layer that is thicker than e.g. 100 μmprevents this. To reproduce the information from the (magneto) opticaldata storage medium, light is also radiated by the optical systemthrough the transparent substrate. In this situation, the optical headis also arranged on the transparent substrate side of the disk.

For objective lenses in optical heads, either slider-based (see FIG. 4)or actuator-based (see FIG. 1A), having a small working distance(typically less than 50 μm), contamination of the optical surface of theobjective closest to the storage medium occurs. This is due tore-condensation of water, which may be desorbed from the storage mediumbecause of the high surface temperature (approximately 250° C.)resulting from the high radiation beam power and temperature requiredfor writing data in (or even reading data from) the recording stack, seeFIG. 3. The contamination ultimately results in malfunctioning of theoptical storage system due to distortion of, for example, system controlsignals, see FIG. 2. The problem becomes more severe for the followingcases: high humidity, high radiation beam power, low opticalreflectivity of the recording stack of the data storage medium, lowthermal conductivity of the storage medium, small working distance andhigh surface temperature. From patent U.S. Pat. No. 6,069,853 it isknown to increase the distance between the recording stack and the outeroptical surface of the storage medium using a thermal insulator or coverlayer, in order to prevent contamination of the objective. Without suchinsulator layer contamination from the medium may evaporate and condenseonto the objective of the optical head during recording. Thecontamination may e.g. be water mixed with small quantities of othercontaminants. The water including other contaminants is probably presentas a thin (mono)layer on the outer surface of the medium. When noinsulator layer is present the thin (mono)layer is present very close tothe recording stack and is indirectly heated by the recording stack,evaporates and subsequently condenses onto the objective including theother contaminants. This occurs relatively rapidly, i.e. within half anhour or possibly less, and causes unreliable recording and reading ofthe system and finally may lead to total recording and reading failure.An advantage of the application of the insulator or cover layer is theprevention of heating of the (mono)layer with contaminants and hence thebuild-up of contamination onto the objective. This is because theinsulator layer forms an effective barrier which prevents the(mono)layer on the medium to be heated and evaporated. However suchinsulator layer, typically with a thickness of tens of nanometers ormore, has several disadvantages. It may, e.g., cause unreliablerecording and read out of data due to optical aberrations andinterference of the insulator layer. Furthermore it may occur that theoptical head focuses on the outer surface of the insulator layer inwhich data recording and read out is impossible after which event theoptical head needs to refocus onto a subsequent surface. This proceduremay lead to interruption of data streams and therefore unreliable datarecording and reading. And furthermore, relatively thick insulator layerrequires the MO magnetic coil to have a relatively large magnetic fielddistance range in axial direction of the coil, which limits theswitching speed of the coil and thus the recording reliability at largerdata rates.

It is an object of the invention to provide a system of the kind asdescribed in the opening paragraph, which performs reliable recordingand readout of data in the recording stack and prevents contamination ofthe objective of the optical head without the mentioned disadvantages.

It is a second object of the invention to provide an optical datastorage medium for reliable recording and readout of data for use in asystem of the kind as described in the opening paragraph.

These objects are achieved in accordance with the invention by anoptical recording and reading system which is characterized in that atleast one of the first optical surface and the second optical surface isprovided with a hydrophobic layer, that has a refractive index n and hasa thickness smaller than 0.5 λ/n. The first new insight is thatre-condensation of water on the optics can be prevented by applying arelatively thin, transparent hydrophobic layer on the objective. Thesecond new insight is that applying a thin hydrophobic layer on therecording stack of the optical data storage medium will prevent waterfrom adsorbing in the first place, so that it cannot be desorbed later.Hence, the source for contamination is eliminated. When the thickness ofthe hydrophobic layer is kept below said limit optical aberrations andinterference are counteracted. An important feature of the technique isthat subsequent cleansing of the objective is not necessary. Also therange of humidity levels in which an optical drive operates reliably maybe improved. Preferably the second optical surface is provided with ahydrophobic layer that has a thickness substantially equal to 0.25 λ/nin which case it may act as an anti reflection coating. In practice itmay be difficult to have a sufficiently hydrophobic layer on theobjective. In that case the second optical surface preferably isprovided with a hydrophylic layer that has a thickness substantiallyequal to 0.25 λ/n. When using a hydrophylic layer, water which may becollected on the second optical surface forms a layer of substantiallyhomogeneous thickness, which may e.g. be about a micron thick. This isbecause the surface wetting is homogeneous. The central portion of thefluid layer, i.e. the portion through which the radiation beampropagates, does not substantially disturb the wavefront of the focusedradiation beam. Hydrophylic layers may be achieved by layers of whichthe hydrophylic properties typically result from oxygen atoms at thesurface such as the case for Al₂O₃— or SiO₂ layers. Mostly the NA insuch low FWD system is larger than 0.80.

In an embodiment the optical head further comprises a magnetic coilarranged at a side of the optical head closest to the recording stacksuch that an optical axis of the optical head traverses the center ofthe magnetic coil and the recording stack of the optical data storagemedium is of the magneto-optical type. In this case reliable magnetooptical recording is possible at a high density and data rate because ahigh NA, i.e. a small spot, is possible and the magnetic coil may bebrought close to the recording stack in which case a magnetic field maybe modulated in an energy efficient way.

It is especially advantageous when the magnetic coil has an innerdiameter of smaller than 60 μm. The use of a bigger coil wouldcompromise the data rate and energy efficiency because bigger coils havea large inductance and higher power consumption.

In an embodiment the hydrophobic layer comprises a material selectedfrom the group of poly-para-xylylenes, fluorocarbons and copolymersthereof. Parylene is a generic name for a family of poly-para-xylylenes.Four different types are commercially available: Parylene-N is the mostbasic form, build as a linear chain of para-xylylene monomers. Othertypes are parylene-C, parylene-D and parylene-AF4. Notwithstanding theapplicability of any dervative of parylene, parylene-C is of particularinterest for our purpose. Another suitable material is AF1600, made byDupont, is a copolymer of tetrafluoroethylene andperfluoro-2,2-dimethyl-1,3-dioxole is especially suited.

In an embodiment the magnetic coil is contained in a slider, that isadapted for flying at a distance of >0.5λ/n and <2 μm from the firstsurface. In this case the slider forms part of the objective and thehydrophobic layer is present at the surface of the slider facing theoptical data storage medium. Slider technology is e.g. known from harddisk drive (HDD) technology. With this technology “near field”configuration is possible in which case the outer surface of objectiveand the outer surface of the optical data storage medium are spaced fromeach other a distance of the order of much less than one wavelength λi.e. FWD ≦λ/10. In such configuration coupling between the medium andthe objective may be effected by evanescent wave optical coupling. TheNA in such configuration can be greater than unity. Far field, i.e.FWD >>λ/10, configuration is also possible with slider-based technology.

In an embodiment the optical data storage medium the first opticalsurface is provided with a transparent hydrophobic layer that has arefractive index n and has a thickness smaller than 0.5 λ/n. When thethickness of the hydrophobic layer is kept below said limit opticalaberrations and interference are counteracted. An important feature ofapplying the hydrophobic layer on the medium is that the necessity of ahydrophobic layer on the objective may be smaller because the source ofcontamination, i.e. the contamination on the medium, is eliminated orgreatly reduced. This has the advantage that older optical recordingsystems which lack any measures taken to minimize the effect ofcondensation, e.g. a hydrophobic layer, on the objective of the opticalhead still benefit from the hydrophobic layer on the medium because themedium causes no or much less contamination. Also the range of humiditylevels in which an optical drive operates reliably may be improved.Preferably the first optical surface is provided with a hydrophobiclayer that has a thickness smaller than 0.25 λ/n in which case even lessoptical aberrations and interference are present.

Preferably the hydrophobic layer on the medium comprises a materialselected from the group of poly-para-xylylenes, fluorocarbons andcopolymers thereof.

It is advantageous to use a material with a relatively high hardness inorder to prevent damage of the hydrophobic layer due to possible contactof the optical head with the hydrophobic layer. When at least one of themedium and the objective has a hydrophobic layer, damage due to shearforces is greatly counteracted because of the extremely low coefficientof friction between the two layers.

The invention will be elucidated in greater detail by means of exemplaryembodiments and with reference to the accompanying drawings, in which:

FIG. 1A shows an embodiment of the system according to the inventionwith small free working distance optics used in an MO drive;

FIG. 1B shows the structure of the layer stack of the medium of FIG. 1A;

FIG. 2 shows oscilloscope traces before and after contamination;

FIG. 3 shows microscope pictures of the contamination of the objectiveas a function of time;

FIG. 4 shows a slider-based optical recording system;

FIG. 5 shows a transparent slider with Magnetic Field Modulation coil;

FIG. 6 shows a microscope picture of the second optical surface of theobjective with an insufficiently hydrophobic layer,

FIG. 7 shows a microscope picture of the second optical surface of theobjective without a hydrophobic layer, but instead with a veryhydrophylic layer.

In FIGS. 1A and 1B, an embodiment is shown of an optical recording andreading system for use with an optical data storage medium 5. The medium5 comprises a recording stack 9 formed on the substrate 8 by e.g.sputtering. The recording stack 9 is suitable for recording by a focusedradiation beam 1. The wavelength λ of the focused radiation beam 1 is405 nm. The recording stack 9 has a first optical surface 6 most remotefrom the substrate 8. An optical head 3, with an objective 2, having anumerical aperture NA=0.85, from which the focused radiation beam 1emanates during recording is present at the recording stack 9 side ofsaid optical data storage medium 5. The objective 2 of the optical head3 has a second optical surface 7 closest to the recording stack 9 and isadapted for recording/reading at a free working distance d_(F)=15 μmfrom the first optical surface 6 of the medium 5. It is noted that thetransparent element which contains a magnetic coil 4 forms part of theobjective 2 and that the second optical surface 7 is the surface of thiselement closest to the medium 5. The first optical surface 6 and thesecond optical surface 7 are provided with a hydrophobic layer 10, 11made of AF 1600, a copolymer of tetrafluoroethylene andperfluoro-2,2-dimethyl-1,3-dioxole, which has a refractive index ofabout 1.35. The thickness of the hyfrofobic layer is 30 nm. Thepreparation of fluoropolymer coatings such as AF1600 is done as follows.Depending on specific circumstances, a cleansing step of the disksurface may be required, consisting of sonification, rinsing and drying.When the disk is clean, a layer of fluorosilane is pre-deposited fromthe vapor phase at room temperature to improve adhesion of thefluoropolymer coating. The fluoropolymer coating can be made by means ofdip coating or spin coating. To this end, the fluoropolymer, such asAF1600, can be dissolved in FC-75 (perfluoro-2-butyltetrahydrofuran,made by Acros). Finally the coated disk is dried in by air in a laminarflow. A thermal treatment is also possible at this point. Coating a diskwith parylene, requires it to be thoroughly clean. On metal surfaces,parylene usually shows good adhesion, but on oxidic surfaces such asglass or aluminium oxide the adhesion is usually less. In those cases alayer of A174 (gamma-methacryl-oxypropyl-trimethoxysilane) is depositedfirst, either from the vapor phase at room temperature or by spin or dipcoating in a 1:100:100 A174:water:iso-propanol mixture. Excess A174 isrinsed away with pure isopropanol, and the disk is dried at ambienttemperature in clean air. Parylene deposition is performed in acommercially available parylene coating machine such as the PDS2010 (ascan be purchased from SCS Europe). Essentially the parylene monomer isvaporized and deposited on the rotating substrate. Further, generallyknown, details can be found in literature. The optical head 3 furthercomprises a magnetic coil 4 arranged at a side of the optical head 3closest to the recording stack 9. An optical axis of the optical head 3traverses the center of the magnetic coil 4 and the recording stack 9 ofthe optical data storage medium 5 is of the magneto-optic type. Therecording stack 9 may e.g. include, in this order starting at thesubstrate 8, a reflective metal layer as known in the art e.g. 25 nm Al,other auxiliary layers, a 24 nm layer of the magnetic material TbFeCoand a 60 nm interference layer of SiN or ZnS/SiO₂.

When the medium 5 does not have a hydrophobic layer 10, a water(mono)layer mixed with contamination will build up on the first opticalsurface 6. When using an objective without hydrophobic layer 11 on thesecond optical surface recording for recording on such a medium, therecording system will in a matter of minutes show servo, e.g. focus ortracking, failure. This is illustrated in FIG. 2. FIG. 3 shows thecontamination build-up at the second optical surface 7 when nohydrophobic layers 10, 11 are used. With at least one hydrophobic layer10 at the first optical surface 6 the system proves to be stable androbust.

In FIG. 2 oscilloscope traces 22 and 23 are shown with the error signalsof the radial open loop and closed loop focus servos respectively fortwo different laser beam powers P. The optical system is in focus, butthe tracking servo has not been closed. At low power (P=0.5 mWatt),signals are normal. At higher power (P=1.0 mWatt) serious instability ofthe signals occurs due to contamination build-up at the second surface7.

In FIG. 3 microscope photos of the second optical surface 7 of theobjective 3 are shown without hydrophobic layer 11 and when is it stillclean (t=0 min), and after contamination build-up after 1 min ofrelatively high laser power recording. Water is clearly visibleindicated by reference numeral 30. The objective was left under themicroscope to dry and observed again after 12 and 40 minutes. At t=40min the water has completely evaporated and contaminants indicated byreference numeral 31 are visible. The contaminants are redepositions ofthe evaporated water/contamination mixture layer on the first opticalsurface 6 of a medium 5 which does not have a hydrophobic layer 10.Application of a hydrophobic layer 10, 11 on respectively the firstoptical surface 6 and the second optical surface 7 of the medium 5and/or the objective 2 greatly counteracts the problem of contaminationbuild-up because no contamination build-up could be observed at moderatelaser power.

In FIG. 4 a slider-based optical recording system is shown. The flyingheight of the slider 2 a above the disk is approximately 1 μm. Note thatthis is not near field recording as defined earlier because 1 μm>>λ/10.In this example, the slider contains a Magnetic Field Modulation (MFM)coil 4, which is used for Magneto-Optical recording. Further referencenumerals correspond to those in FIG. 1A. It is noted that the portion ofthe slider 2 a through which the focused radiation beam 1 traversesforms part of the objective 2.

In FIG. 5 a transparent slider with Magnetic Field Modulation coil isshown. The laser beam of FIG. 4 is focused through the aperture in themiddle.

In FIG. 6 a microscope photo of the second optical surface 7 of theobjective 3 is shown with an (insufficiently) hydrophobic layer 11.Insufficient refers to the value of the contact angle between a waterdroplet and the hydrophobic layer, which in this case is not much largerthan 90 degrees, but which for an ideally hydrophobic layer is 180degrees or close thereto. Due to the fact that the contact angle betweenwater and surface is not much larger than 90 degrees, clearly some smalldroplets are formed and visible, as indicated by reference numeral 60.This cluster of droplets has a size both comparable to the free workingdistance between objective and recording medium and the diameter of thefocused laser beam. The laser beam will be scattered substantially bythe cluster of water droplets. The free working distance is the distancebetween the first optical surface 6 and the second optical surface 7.

In FIG. 7 a microscope photo of the second optical surface 7 of theobjective 3 is shown without a hydrophobic layer, but instead with avery hydrophylic layer. The hydrophylic properties typically result fromoxygen atoms at the surface such as the case for Al₂O₃ or SiO₂. Thecontact angle between liquid (water) and surface is now very small, i.e.much smaller than 90 degrees. As can be seen in the photo, a full layerof water with substantially homogeneous thickness is collected on thesurface, indicated by reference numeral 70, as compared to FIG. 6 andthe layer is about a micron thick and may even be thicker. Because thesurface wetting is homogeneous, at least the central portion of thefluid water layer has an optically constant thickness and does notsubstantially disturb the wavefront of the focused laser beam.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

According to the invention an optical recording and reading system foruse with an optical data storage medium is described. The systemcomprises the medium having a recording stack, formed on a substrate.The recording stack is suitable for recording by means of a focusedradiation beam with a wavelength λ in air. The recording stack has afirst optical surface most remote from the substrate. An optical head,with an objective having a numerical aperture NA and from whichobjective the focused radiation beam emanates during recording, isarranged on the recording stack side of said optical data storagemedium. The objective has a second optical surface closest to therecording stack, and is adapted for recording/reading at a free workingdistance d_(F) of smaller than 50 μm from the first optical surface. Atleast one of the first optical surface and the second optical surface isprovided with a transparent hydrophobic layer that has a refractiveindex n and has a thickness smaller than 0.5 λ/n. In this way reliablerecording and reading is achieved, specifically contamination build-upon the second optical surface is prevented or counteracted.

1. An optical recording and reading system for use with an optical datastorage medium (5), said system comprising: the medium (5) having arecording stack (9), formed on a substrate (8), said recording stacksuitable for recording by means of a focused radiation beam (1) with awavelength λ in air, the recording stack having a first optical surface(6) most remote from the substrate (8); and an optical head (3), with anobjective (2) having a numerical aperture NA and from which objective(2) the focused radiation beam emanates (1) during recording, theobjective (2) arranged on the recording stack (9) side of said opticaldata storage medium (5) and having a second optical surface (7) closestto the recording stack (9), and adapted for recording/reading at a freeworking distance d_(F) of smaller than 50 μm from the first opticalsurface (6), characterized in that at least one of the first opticalsurface (6) and the second optical surface (7) is provided with atransparent hydrophobic layer (10) that has a refractive index n and hasa thickness smaller than 0.5 λ/n.
 2. A system according to claim 1,wherein the second optical surface (7) is provided with a hydrophobiclayer (11) that has a thickness substantially equal to 0.25 λ/n.
 3. Asystem according to claim 1, wherein the second optical surface (7) isprovided with a hydrophylic layer (11) that has a thicknesssubstantially equal to 0.25 λ/n.
 4. A system according to claim 1,wherein the optical head (3) further comprises a magnetic coil (4)arranged at a side of the optical head (3) closest to the recordingstack (9) such that an optical axis of the optical head (3) traversesthe center of the magnetic coil (4) and the recording stack (9) of theoptical data storage medium (5) is of the magneto-optical type.
 5. Asystem according to claim 4, wherein the magnetic coil (4) has an innerdiameter smaller than 60 μm.
 6. A system according to any one of claims1-5, wherein the hydrophobic layer (10, 11) comprises a materialselected from the group of poly-para-xylylenes, fluorocarbons andcopolymers thereof.
 7. A system according to any one of claims 4-6,wherein the magnetic coil (4) is contained in a partially transparentslider, that is adapted for flying at a distance of >0.5 λ/n and <2 μmfrom the first surface (6).
 8. An optical data storage medium (5) havinga recording stack (9), formed on a substrate (8), said recording stacksuitable for recording by means of a focused radiation beam (1), with awavelength λ in air, the recording stack having a first optical surfacemost remote from the substrate, characterized in that the first opticalsurface (6) is provided with a transparent hydrophobic layer (10) thathas a refractive index n and has a thickness smaller than 0.5 λ/n.
 9. Anoptical data storage medium according to claim 8, wherein the firstoptical surface is provided with a hydrophobic layer (10) that has athickness smaller than 0.25 λ/n.
 10. An optical data storage medium (5)according to claim 8 or 9, wherein the hydrophobic layer comprises amaterial selected from the group of poly-para-xylylenes, fluorocarbonsand copolymers thereof.
 11. Use of an optical data storage medium (5)according to anyone of claims 8-10 for reliable recording and reading ina system as claimed in anyone of claims 1-5.